Characterisation of a Commercial Automotive NH₃-SCR Copper–Zeolite Catalyst

A commercial copper exchanged zeolite was characterised and studied regarding the selective catalytic reduction of nitrogen oxides (NO_X) by ammonia using physicochemical analyses (XRF, XRD, NMR, BET, UV–Visible, IR) and synthetic gas bench test rig, respectively. As expected, two adsorption sites were identified for ammonia: Brönsted and Lewis acid sites, the latter retaining stronger ammonia. Furthermore, ammonia and water adsorb on the same sites, and competition phenomena decrease ammonia storage capacity. Concerning the NO_X conversion, in spite of a high efficiency, an important selectivity into N₂O is noticed, due to the formation of an ammonium nitrate by-product on the catalyst surface. The limited NO_X conversion efficiency at low temperature is due to the weak NO oxidation activity, whereas NH₃ oxidation activity at high temperature involves a decrease in NO_X reduction.